1. Field of the Invention
The present invention relates to an image forming apparatus forming images by an electrostatic duplicating process, such as a copier, a facsimile and a printer. Further, the present invention relates to a process cartridge installed therein, an image forming method and a developer for electrophotography applied therefor.
2. Discussion of the Background
In an electrophotographic image forming apparatus such as a copier or a printer, an image bearer uniformly charged is irradiated to form a latent image thereon; the latent image is developed with a toner to form a toner image; and the toner image is transferred onto a transfer material such as a recording paper. The transfer material bearing the toner image passes through a fixer wherein the toner image is fixed thereon upon application of heat or pressure.
In the image forming apparatus, an image developer developing the latent image on the image bearer uses a one-component developing method using a toner including a magnetic material or a two-component developing method using a developer including a toner and a carrier.
The image developer using the two-component developing method has good developability and is used for most of the image forming apparatuses currently used. Particularly in recent years, many color-image forming apparatuses forming full-color or multi-color images are used, and demand for the image developer using the two-component developing method is further increasing.
The toner and carrier are stirred in the image developer using the two-component developing method, and the toner is frictionally-charged with the carrier and electrostatically attracted to the outer surface of the carrier. The carrier bearing the toner is transported to a developing area where the toner leaves from the carrier and electrostatically adheres to the latent image on the image bearer upon application of developing bias to form a toner image. Therefore, in the two-component developing method, it is essential that the carrier stably charges the toner when stirred before and after used for long periods to produce images satisfying high durability and stability.
In the typical image developer using a two-component developer, a toner is consumed and a carrier remains therein in the mean time while developing images. Therefore, the carrier being stirred with the toner deteriorates as it is more frequently stirred therewith because a resin coated on the carrier peels and the toner adheres thereto. Accordingly, the resistivity of the carrier and the chargeability of the developer gradually deteriorate, and the developability of the developer excessively increases. Resultantly, image density excessively increases and foggy images are produced.
In order to solve this problem, Japanese Published Examined Patent Application No. 2-21591 discloses a trickle image developer wherein a carrier is gradually replaced while a toner is consumed for developing images to prevent variation of the charge quantity of the developer for stabilizing the image density.
However, even in the image developer disclosed in Japanese Published Examined Patent Application No. 2-21591, deteriorated carrier gradually increases as the developer is used for a long time and it is difficult to prevent increase of the image density.
Japanese Published Unexamined Patent Application No. 3-14 5678 discloses a supplemental developer to be properly fed in the image developer, wherein a carrier has a higher resistivity than that of a carrier readily contained in the image developer to maintain the chargeability and prevent deterioration of image quality.
Further, Japanese Published Unexamined Patent Application No. 11-223960 discloses a supplemental developer including a carrier imparting higher charge quantity to a toner to maintain the chargeability and prevent deterioration of image quality.
However, the carrier quantity replaced in the image developer differs with the difference of the toner consumption, the resistivity or charge quantity of the developer disclosed in Japanese Published Unexamined Patent Applications Nos. 3-145678 and 11-223960 varies, resulting in variation of image density.
Japanese Published Unexamined Patent Application No. 8-234550 discloses a method of sequentially feeding plural developers including carriers having different properties from those of a carrier readily contained in an image developer.
However, practically, it is quite difficult to feed sequentially feeding the plural developers including carriers having different properties in the image developer so as not be mixed with each other because the specific gravities of a toner and a carrier are extremely different from each other. In addition, the carrier tends to deteriorate because the toner quantity is too large for the carrier in the developer, and which does not produce images having stable quality.
As disclosed in Japanese Published Unexamined Patent Application No. 8-234550, when silicone-coated layer coated on a core material of the carrier is simply increased to increase the resistivity of the supplemental carrier, the charge quantity of the carrier decreases although the resistivity thereof increases, resulting in deterioration of reproducibility of images and occurrence of background fouling.
Therefore, in the trickle developing method, it is essential that the carrier can maintain stable chargeability even when used for long periods.
Japanese Published Unexamined Patent Application No. 58-108548 discloses coating a granulated carrier for use in a two-component developer with a proper resin for the purpose of preventing a toner from filming over the carrier, forming a uniform surface thereof, preventing the surface thereof from being oxidized, preventing deterioration of moisture sensitivity thereof, extending a life of the developer, protecting a photoreceptor from being scratched or abraded with the carrier, controlling a charge polarity, adjusting charge quantity, etc; and Japanese Published Examined Patent Applications Nos. 1-19584 and 3-628, and Japanese Published Unexamined Patent Application No. 6-202381 disclose a method of adding various additives to the coated layer.
Further, Japanese Published Unexamined Patent Application No. 5-273789 discloses a carrier, the surface of which an additive adheres to, and Japanese Published Unexamined Patent Application No. 9-160304 discloses a carrier including an electroconductive particulate material larger than the thickness of a coated layer thereof.
Japanese Published Unexamined Patent Application No. 8-6307 discloses using a carrier coating material mainly including a benzoguanamine-n-butylalcohol-formaldehyde copolymer, and Japanese Patent No. 2683624 discloses using crosslinked material between a melamine resin and an acrylic resin as a carrier coating material.
However, even these carriers still have problems in their durabilities or heat resistances, and problems spent carrier, unstable charge quantity and foggy images. Further, the environmental resistance needs improvement.
In addition, a resistivity adjuster is conventionally included in a carrier in a two-component developer to have stable chargeability. Carbon black is mostly used as the resistivity adjuster.
However, when such a carrier is used in a color image forming apparatus, the surface of the carrier is abraded or carbon black leaves therefrom and transfers in color images, resulting in possible color contamination.
Various methods are disclosed to prevent this phenomenon.
For example, Japanese Published Unexamined Patent Application No. 7-140723 discloses a carrier wherein an electroconductive material (carbon black) is present on the surface of a core material and not in a coated layer.
Japanese Published Unexamined Patent Application No. 8-179570 discloses a carrier having a concentration gradient of carbon black in its coated layer, wherein the concentration becomes lower toward the surface thereof and carbon black is not present at the surface thereof.
Japanese Published Unexamined Patent Application No. 8-286429 discloses a double-coated carrier wherein an inner coated layer including electroconductive carbon is formed on the surface of a core material and a coated layer including a white electroconductive material is formed thereon.
However, recently, electrophotographic image forming apparatus is noticeably required to form an image at higher speed, and a developer receives stress more and more. Therefore, it is difficult to completely prevent the color contamination caused by transfer of carbon black in images even with the carriers disclosed in Japanese Published Unexamined Patent Applications Nos. 7-140723, 8-179570 and 8-286429.
Japanese Published Unexamined Patent Application No. 2004-29306 discloses a feeder feeding a supplemental developer.
However, higher speed and higher quality need a uniform concentration of a toner and a carrier. When a large amount of the carrier is fed into a developer tank, the supplemental developer is late in discharging from the developer tank and the amount of the developer therein temporarily increases. When the amount of the developer therein increases, e.g., when the toner concentration is controlled by detecting a magnetic permeability, the developer fed per unit of time a magnetic permeability sensor detects increases. The density of the developer at the detection surface increases and the magnetic permeability sensor indicates a high magnetic permeability. Therefore, the supplemental developer is further fed to maintain an initial standard, resulting in feeding of the developer having a toner concentration higher than the standard. On the contrary, when the amount of the developer therein decreases, the developer having a toner concentration lower than the standard is fed. Thus, the image density varies. Japanese Published Unexamined Patent Application No. 2004-29306 specifies the specific gravity of the supplemental developer. A carrier having low specific gravity somewhat improves the uniformity of concentrations of the toner and carrier. However, a carrier having high specific gravity does not. In addition, the carrier having high specific gravity is likely to cause an excessive supply (flashing) of the toner, resulting in scattering of the toner from feeding route thereof.
Because of these reasons, a need exists for an image forming apparatus using a two-component developer, stably producing high-quality images, having high durability, preventing a carrier from adhering to a solid image, and producing no contaminated color image.